Amplifier circuits



Nov. 20,

PR5 SHEEN? 1934. w. VAN B. ROBERTS 1,981,071

AMPLIFIER CIRCUITS Filed April 14, 1931 H W a M 1| 2 R L 5 U l/IYZ/M/fll 1 M54545 g 2-8 I "1 5 T INVENTOR WALTER VAN B ROBERTS ATTORNEY Patented Nov. 20, 1934 UNITED STATES AMPLIFIER CIRCUITS Walter van B. Roberts,

Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 14, 1931, Serial No. 529,931

10 Claims.

My present invention relates to electrical amplifier circuits, and more particularly to amplifier circuits adapted to amplify radio frequency currents without effects due to the flow of retroactive currents.

One of the main. objects of my present invention is to provide an electrical amplifier circuit employing an electron discharge tube having a high mutual conductance, and which circuit is adapted to amplify radio frequency currents without effects due to the flow of regenerative currents arising out of coupling between the output and input circuits associated with such atube, the output circuit of the tube including a plurality of controls for regulating the selectivity and voltage amplification of the amplifier circuit in any predetermined manner.

Another important object of the present invention is to provide in a radio receiver, a radiofrequency amplifier circuit including an electron discharge tube having a high mutual conductance, the input of the tube being connected to a frequency selection device, and the output of the tube including a coupling arrangement having a series resonant path in the anode circuit of the tube, the path including a variable capacity and a variable inductance, there being a variable impedance in shunt with said series resonant path to control the selectivity of the amplifier circuit in any predetermined manner.

Still other objects of the invention are to improve generally the efficiency of radio frequency amplifier circuits, and to particularly provide in such a circuit, a coupling arrangement which is not only reliable in operation, but capable of regulation in any desired manner.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of =operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit arrangements whereby my invention may be carried into effect.

Referring to the accompanying drawing there is diagrammatically shown a radio receiver comprising signal energy collecting means A, connected to a frequency selection device. The latter is grounded, as at G, and is conventionally represented, the conventional representation being denoted by the term Pre-Selector. To preserve simplicity of description the detailed construction of this pre-selector device is not disclosed, it being well known a those skilled in the art that a plurality of tuned radio frequency amplifier stages connected in cascade may be utilized for the preselector, or, in general, coupled resonant circuits may be employed to select a band of signal frequencies to be amplified and detected in there"- ceiver.

For example, the well known'type of band pass selector circuit comprising a pair of coupled resonant circuits, whose variable reactances are unicontrolled, may be utilized in the'last mentioned type of pre-selector device. In any case, the preselector' device will include some arrangement whereby with-respect to broadcast reception, radio frequency signals will be selected and impressed upon the input electrodes of the amplifier circuit to be described.

According to my present invention, an electron discharge tube 1 has its control electrode and cathode connected to the output of the aforementioned pro-selector device. The control electrode circuit may include a grid biasing source C and tube 1 preferably includes a space charge electrode 3 disposed between cathode and control grid electrode. The anode of the tube isconnected to the positive terminal of a source of anode potential B through a variable resistor R, while the space charge electrode 3 is connected to the positive terminal of a source S. Of course,'it'is well understood that the sources S and B can be a common source.

While the tube 1 has been specifically shown as a four electrode tube including a space charge electrode, it is pointed out that the tube-need not be of this specific type. The essential requirement for the structure of tube 1 is that the tube have a high mutual conductance. In other words, any construction may be'utilized for the tube 1 which will result in a device having a high mutual conductance.

In shunt with the variable resistor B, there is provided a series resonant path comprising avariable capacity C and a variable inductance' L, the high potential side of the capacity being connected to one end of the resistor R, 'while the low potential side of the inductance L is connected to the cathode of tube 1. After the selected energy has been amplified in the stage including the tube 1 and the aforementioned coupling arrangement, 7

the amplified energy is impressed across the input ergy is secured in a manner well known to. those skilled in the art.

device 2 need not necessarily be employed with the output of one stage of amplification including a tube 1, but that several stages of amplification as disclosed herein may be employed in cascade between the pre-selector device and the detector device. 9

. the output circuit of the amplifier tube 1 a cou- It will now be seen that there is provided in pling arrangement which includes three adjustable elements, to wit: a variable impedance,

specifically a resistor R, a variable capacity C,

and a variable inductance L. I'have found that these three adjustable impedances may be "controlled independently or conjointly, to secure va rious relationships between amplification and selectivity throughout a predetermined tuning range. Furthermore, by means of the arrangement shown, it is possible to secure amplification of a selected band of radio frequency energy without the production of efiects due to the flow of reactive or regenerative currents through capacitive coupling between the output and input circuits of tube 1.

. Such capacitive coupling has been conventionally represented, in dotted lines, by the reference letter F, the feedback path being represented as including inherent capacity between the anode and control electrode of the tube 1, it being clearly understood that such a path Falso represents connections and circuits associated with the anode and control electrode of the tube 1. No feedback occurs through the path F, for the reason that the radio frequency potential of the anode of tube 1 is negligible, due to the fact that there is disposed a series resonant path in the anode circuit of the tube.

Without limiting my invention to depend upon any particular theory, it is believed that the oper ation of the present amplifier circuit may be explained asfollows: At resonance, the current flowing through the inductance L can be expressed by the following equation:

R IF *G e where Gm represents the mutual conductance of the tube 1; and, r is the resistance of the series resonant circuit L, C.

The voltage amplification, at resonance, may ,be expressed by the following equation:

If R becomes equal to r, the voltage amplification at resonance may be expressed by thejfollowing equation: v n

E=%G 21rfL.

Considering a particular value for the mutual lcond'uctance of the tube 1, if Gm is equal to 10- therejfwould be, obtained. a voltage amplification of l5',"if 21r L equals three thousand. It is obvious that large values of mutual conductance are necessary to obtain any considerable amplification.

The selectivity of the stage of amplification including the tube 1 and the coupling arrangement in the output circuit of the tube may be Varied by regulation of the resistor R. For example, the selectivity may be reduced by increasing the value .of B. At the maximum value of the resistor R,

there will be very little selectivity, a very broad band being secured. At the minimum setting of the resistor R, that is to say where R is approximately equal to r, ood selectivity is obtained, as Well as good amplification. By coupling, in any desired mechanical manner, the three variable elements R, C and L, it is possible to secure any desired variationfof selectivity and amplification throughout a predetermined tuning range.

The mechanical coupling between the three impedances is designated by the reference letterU, the coupling being conventionally represented by dotted lines. This coupling may be operated by an independent control knob 6,but' it is preferable to mechanically couple the, three adjustable impedances for uni-control with the control instrumentalities of the pro-selector device. "As a specific example of a. type of variation which may be secured by means of the present coupling arrangement, it is possible to have the inductance vary inversely with frequency, and variableresistor R decreases in value as the fre 'quency to which the series resonant path C, L tunes, increases in value. This will result in the maintenance of a constant amplification characteristic throughout a predetermined tuning range in spite of increasing radio frequency resistance of coil L.

In general, it is obvious that with a possibility of adjusting R, L and Gas independent variables, then since. selectivity, amplification, and resonant frequency are each functions of, R, L, and C, values of R, L and C can always beqchosen to yield any given amplification, selectivity and resonant'f'requency. That is, three simultaneous equations determine the values of three independent variables. Of course, in practice, the above is subjecttolimitationin that the valuessodetermined mayfall outside the range of values attainable in the actual instruments used. Finally, it is also possible to uni-control the variation of the constants so as to make the amplification, selectivity and frequency vary in predetermined fashion with movement of the uni-control handle.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparenttorone skilled in the art that my invention is by nomeans limited to the particular organization shown and described, but that many modifications in the circuit arrange ments, as well as in the apparatus employed, may

be made without departingfrom thescope of my invention as set forth in the appended claims. What I claim is: q 1. An electrical amplifier circuit comprising, in combination, a space discharge device including at least an anode,,a cathode, a control electrode and having a high mutual conductance, an input circuit for said tube associated with said cona constant high amplification characteristic is,

maintained throughout a predetermined resonance range of said path.

2. An electrical amplifier circuit comprising, in combination, a space discharge device including at least an anode, a cathode, a control electrode and having a high mutual conductance, an input circuit for said tube associated with said control electrode, an anode circuit connected between said anode and cathode and including a series resonant path in series in the anode circuit, and a variable resistance in shunt with said path, said path comprising a variable capacity and a va riable inductance in series, said variable elements being designed to maintain a uniform high amplification characteristic throughout a wide range of variation of the condenser.

3. An electrical amplifier circuit comprising, in combination, a space discharge device including at least an anode, a cathode, a control electrode and having a high mutual conductance, an input circuit for said tube associated with said control electrode, an anode circuit connected between said anode and cathode and including a series resonant path in series in the anode circuit, and a variable resistance in shunt with said path, said path comprising a variable capacity and a variable inductance in series, and a common means for regulating said resistance, capacity and inductance, said inductance being arranged to vary inversely with frequency change of said path, and said resistor decreasing in value as said frequency increases.

a. An electrical amplifier circuit comprising, in combination, a space discharge device including at least an anode, a cathode, a control electrode and having a high mutual conductance, an input circuit for said tube associated With said control electrode, an anode circuit connected between said anode and cathode and including a series resonant path in series in the anode circuit, and a variable resistance in shunt with said path, said path comprising a variable capacity and a variable inductance in series, and a common means for regulating said resistor and at least one of the elements in said path, said resistor decreasing in value as the frequency of said path increases.

5. In combination, in an electrical amplifier circuit, an electron discharge tube having a high mutual conductance, a preselector device coupled to the input electrodes of said tube, a series resonant path in the anode circuit of said tube, means for adjusting said path to be resonant with a desired frequency, a variable resistor in shunt with said path, and a common means for adjusting said variable resistor and path adjusting means to secure a predetermined variation of selectivity and amplification in said circuit.

6. An arrangement for coupling the plate circuit of a tube having a large mutual conductance to the input of a second tube, comprising in parallel, between plate and cathode of said first tube, a circuit consisting of a resistor in series with a source of space current, and a circuit including reactance elements of opposite sign whose reactances annul one another at approximately the frequency being amplified, and connections whereby the voltage drop across one of said reactance elements is impressed upon the input of said second tube, the resistance of said resistor being very much lower than the plate resistance of the first mentioned tube.

7. An arrangement for coupling the plate circuit of a tube having a space charge grid and having a large mutual conductance to the input of a second tube, comprising in parallel, between plate and cathode of said first tube, a circuit consisting of a resistance in series with a source of space current, and a circuit including reactance elements of opposite sign whose reactances annul one another at approximately the frequency being amplified, and connections whereby the voltage drop across one of said reactance elements is impressed upon the input of said second tube, said resistance being very much lower than the plate resistance of the first mentioned tube.

8. In combination, a radio frequency amplifier circuit including an electron discharge tube having a high mutual conductance, a pre-selector device coupled to the input electrodes of said tube, a series resonant path in the anode circuit of said tube, means for adjusting said path to be resonant with a desired frequency, a variable resistor in shunt with said path, a common means for adjusting said variable resistor and path adjusting means tosecure a predetermined variation of selectivity and amplification in said circuit, and a load circuit including a tube having its input circuit connected across an inductive portion of said path.

9. A radio frequency amplification system associated with the plate circuit of an electron discharge tube having a high mutual conductance and being provided with a cathode, a control grid and plate, said system including a resistor connected between the plate and cathode of said tube, and a condenser and an inductance coil eifectively connected in series with each other and between said plate and said cathode, and a common means for adjusting the resistor, condenser and coil so that a constant amplification characteristic is secured, throughout a required frequency range in spite of increasing radio frequency resistance of said coil.

10. A radio frequency amplification system associated with the plate circuit of an electron discharge tube having a high mutual conductance and being provided with a cathode, a control grid, a positively charged space charge grid and a plate, said system including a resistor connected between the plate and cathode of said tube, and a condenser and an inductance coil eifectively connected in series with each other and between said plate and said cathode, and a common means for adjusting the resistor, condenser WALTER VAN B. ROBERTS. 

